Positron Emission Topography (PET) is a non-invasive medical imaging technique which provides an image of a positron emitting radio pharmaceutical and its distribution in the body on a metabolic level. PET is able to provide information on both structure and function of different parts of the brain. It is becoming an important fundamental tool for both clinical diagnosis and medical research. Presently, PET performance is limited by the photosensors used. The effort proposed here will examine an approach to improve resolution of the PET imaging systems by combining the signals from two types of sensors. A photomultiplier tube is the primary detector, and additional solid state indium iodide (InI) photodetectors are used to obtain better spatial resolution. Our Phase I results with indium iodide (bandgap 2.01eV) as a photodetector are very encouraging. InI photodetectors were produced with high optical quantum efficiency and successfully resolved 511 keV at room temperature when coupled to scintillators such as BGO, LSO and CsI(Tl). Prototype arrays with 2x2 elements were also fabricated and successfully tested. The goal of this Phase II project is to advance the InI detector technology to the extent that they can be used in commercial PET machines. A complete PET sub-assembly with InI photodetector array, BGO scintillator array and a photomultiplier tube will be built and its performance will be extensively evaluated. Based on the Phase I results, we believe that our Phase II research on room temperature InI photodetectors has a high probability of producing a new detector technology with better performance which could be the basis for a future generation of PET instruments. PROPOSED COMMERCIAL APPLICATIONS: InI semiconductor detector with high quantum efficiency would have an enormous impact on the design of PET instruments. In addition, a high performance room temperature scintillation photodetector would bind many other nuclear medical imaging, nuclear physics, environmental monitoring, and space exploration applications. Quality control and non- destructive evaluation would be other possible applications.